I use SourceTree daily and never had such issue. But today I did some minor change and now I ended up having this issue:
What I changed today?
I added p4Merge as merge tool and diff tool in my git config. Then I changed my mind and deleted entries for p4Merge from my /.gitconfig file since I noticed I ended up having double entries for merge and diff tools there.
Neither SourceTree staging nor issuing git add . in terminal will stage files. I have been able to rename file, then git will see that change and I can add, commit, then rename back and add and commit again to fix this issue. But this issue keeps reoccuring, so this fix is not really fixing it.
Here is how that looked

However, after removing all entries for p4Merge tool, I noticed that I am unable to stage files from SourceTree anymore. Below screenshot shows that I have 4 upstaged files. If I try to stage them by checking off the checkbox beside them, nothing will happen. The wont be moved to Staged files section
